1. Field of the Invention
The present invention relates to a zoom lens (magnification varying optical system). In particular, the present invention relates to a zoom lens used on a projection apparatus configured to project an image on a screen.
2. Description of the Related Art
In a conventional negative lead type zoom lens, in which a lens unit having a negative refractive power is located closest to the object side, it is easy to achieve a wide angle of view by providing a relatively long back focus. A negative lead type zoom lens like this has been widely used on a projection apparatus. However, if the negative lead type zoom lens, in which a lens unit having a negative refractive power is located closest to the object side, is to be provided with a long focal length, it becomes necessary to use a large-size lens.
On the other hand, Japanese Patent Application Laid-Open No. 2005-309061 and Japanese Patent Application Laid-Open No. 08-297243 discuss a positive-lead type zoom lens, in which a lens unit having a positive refractive power is located closest to the object side. The zoom lens discussed in Japanese Patent Application Laid-Open No. 2005-309061 and Japanese Patent Application Laid-Open No. 08-297243 includes five lens units including, in order from an enlargement side to a reduction side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power. For variation of magnification, any predetermined lens unit or units of the five lens units are moved.
However, in the zoom lens discussed in Japanese Patent Application Laid-Open No. 2005-309061, the brightness may vary due to variation of the F-number (Fno), which may be caused when the zooming position is changed. In the zoom lens discussed in Japanese Patent Application Laid-Open No. 08-297243, variation of the Fno, which may occur when the zooming position is changed, is suppressed but the amount of distortion occurring during zooming from the wide-angle end to the telephoto end may greatly vary.
More specifically, in the zoom lens according to the above-described conventional methods, variation of the Fno cannot be effectively suppressed at the same time as suppressing the variation of distortion at the wide-angle end and at the telephoto end.